Since this is an outbuilding you have lots of opportunities for dirty wiring. Since it's a distant location I'd try replacing all the cross-connects at each end, choosing a different pair on the buried or overhead feeder, using a different lightning suppression at each end, re-punching any connections you don't replace (both ends of station cable, for example), and changing the base cord. The biggest cause I've seen for this error is oxidized punch downs at 66 blocks, but any of these others could be a candidate.